During operation of a nuclear power reactor, impurities and products of the reactor coolant are deposited on nuclear fuel assemblies. These deposits can impact operation and maintenance of nuclear power plants in a number of ways; for example, (a) their neutronic properties can adversely affect the nuclear performance of the reactor; (b) their thermal resistance can cause elevated surface temperature on the fuel rods that may lead to material failure in the rod; (c) their radioactive decay results in work radiation exposure when they are redistributed throughout the reactor coolant system, in particular during power transients: (d) they complicate thorough inspection of irradiated nuclear fuel assemblies by both visual and eddy current methods; (e) deposits released from fuel rods tend to reduce visibility in the spent fuel pool, significantly delaying other work in the fuel pool during refueling outages; (f) once reloaded into the reactor on assemblies that will be irradiated a second or third time, they form an inventory of material that can be redistributed onto new fuel assemblies in a detrimental manner.
Axial offset anomaly (AOA) has been reported in pressurized water reactors (PWRs). AOA is a phenomenon in which deposits form on the fuel rod cladding due to the combination of local thermal-hydraulic conditions and primary-side fluid impurities characteristic of the reactor and the primary system. These deposits cause an abnormal power distribution along the axis of the core, reducing available margin under certain operating conditions. AOA has forced some power plants to reduce the reactor power level for extended periods.
The problem of AOA has necessitated the development of an efficient, cost-effective mechanism for removing PWR fuel deposits. Such a mechanism is also desirable to reduce total deposit inventory to lower dose rates for plant personnel, to improve fuel inspectability, to prepare fuel for long-term dry storage, and to facilitate the collection of corrosion samples for analysis.
Several approaches have been proposed to remove PWR fuel deposits. One method is to chemically clean assemblies in situ in the reactor, or after being removed to a separate cleaning cell. There are several problems with this approach, including cost, potential for corrosion by the cleaning chemicals, and the difficulty of disposing of the resultant highly contaminated chemicals. Perhaps the greatest shortcoming of this chemical only approach is that it is time consuming, requiring several hours to clean a single fuel assembly.
U.S. Pat. No. 6,396,892 and U.S. Patent Publication No. 2002-0163990 A1, both of which are incorporated herein by reference as if fully written out below, disclose an apparatus and method for ultrasonically cleaning irradiated nuclear fuel assemblies of nuclear power plants. Ultrasonic fuel cleaning is effectively a mechanical method for removing corrosion products from nuclear fuel, using cavitating bubbles from induced boiling. Ultrasonic transducers are combined, typically in a radial configuration, to lower the local pressure waves through the water in the fuel assemblies enough to cause nucleate boiling that collapses immediately afterwards. The collapse of the bubble releases solid corrosion products that are then pumped to a filtration system.
The mechanical cleaning is effective, but it is not 100% efficient because corrosion products remain on the fuel assemblies. It is estimated that ultrasonic cleaning removes up to 80% of the total corrosion product inventory on the fuel. A modified ultrasonic cleaning regimen included ‘pulsing’, or intermittent use of the transducers to enhance the cleaning. This technique had little additional effect on corrosion removal.
The industry had considered options for more complete removal of fuel corrosion via full-system chemical decontamination with fuel in-core. While it is expected that the chemical decontamination with fuel in-core could be safely implemented, the implementation would be lengthy and the process very expensive.